15. The Regulation of Dorsiventral Symmetry in Plants
235
The II primordium subsequently developed into a determinate radially symmetrical
structure indicating that influences from the SAM may be important for acquisition
of ab-adaxiality in developing leaf primordia. Hanawa (1961) tangentially bisected
a young leaf primordium on a Sesamum shoot meristem. The adaxial half developed into a bilaterally symmetrical leaf while the abaxial half formed a radially
symmetrical structure lending further support to the hypothesis about SAM control
of leaf symmetry. In a similar study on Manihot esculenta, M6dard (1988) used
microsurgery to either restrict the contact between adaxial and abaxial regions of
leaf primordia or to remove the more delicate adaxial region of primordia. Various
abnormal structures with radially symmetrical leaves and leaves with more rounded
lobes resulted. It was suggested that contact between the adaxial and abaxial regions of the leaf primordium controls organogenesis of the leaf lamina. In a noninvasive experiment, Fleming and coworkers (1997) placed EXPANSIN-loaded
beads onto the 12 site of tomato shoot apices. The regions where the beads were
placed grew out and formed radially symmetric lateral organs and also often resulted in a reversal in phyllotaxis. These results suggest that biophysical constraints
prevent outgrowths at the shoot apex and that at the 12 site, primordium determination into an organ with ab-adaxility has not yet taken place. However, as discussed
above, radial leaf portions may form postgenitally from initially dorsiventral primordia.
10 Genetic and Molecular Analyses of Asymmetry
in Lateral Organs
Recently, several studies have investigated the genetic controls of symmetry in certain plant organs. The molecular studies have focused on the establishment of
transsectional symmetry in leaves, and radial symmetry in flowers. Also, studies on
the establishment of dorsiventrality in lateral organs and the consequences of this
on lateral organ development in insects and other animals have provided additional
insights into the mechanisms by which gene expression cascades can set up gradients and establish two opposing cell fates.
10.1 Studies in Leaves
Radially symmetric leaves in phan mutants are abaxialized while phab mutant leaves
are adaxialized (Waites et al. 1998; McConnell and Barton 1998). Based on the
phantastica, phabulosa, and lbll mutant phenotypes, we can hypothesize that both
adaxial and abaxial cell fates are necessary for proper margin and lamina development. These results also suggest that adaxial and abaxial ceIl fates are mutually
antagonistic.
PHANTASTICA mRNA accumulates in lateral organ primordia such as leaf
primordia and flower primordia (Fig. 4). Interestingly, this PHANTASTICA mRNA
235
The II primordium subsequently developed into a determinate radially symmetrical
structure indicating that influences from the SAM may be important for acquisition
of ab-adaxiality in developing leaf primordia. Hanawa (1961) tangentially bisected
a young leaf primordium on a Sesamum shoot meristem. The adaxial half developed into a bilaterally symmetrical leaf while the abaxial half formed a radially
symmetrical structure lending further support to the hypothesis about SAM control
of leaf symmetry. In a similar study on Manihot esculenta, M6dard (1988) used
microsurgery to either restrict the contact between adaxial and abaxial regions of
leaf primordia or to remove the more delicate adaxial region of primordia. Various
abnormal structures with radially symmetrical leaves and leaves with more rounded
lobes resulted. It was suggested that contact between the adaxial and abaxial regions of the leaf primordium controls organogenesis of the leaf lamina. In a noninvasive experiment, Fleming and coworkers (1997) placed EXPANSIN-loaded
beads onto the 12 site of tomato shoot apices. The regions where the beads were
placed grew out and formed radially symmetric lateral organs and also often resulted in a reversal in phyllotaxis. These results suggest that biophysical constraints
prevent outgrowths at the shoot apex and that at the 12 site, primordium determination into an organ with ab-adaxility has not yet taken place. However, as discussed
above, radial leaf portions may form postgenitally from initially dorsiventral primordia.
10 Genetic and Molecular Analyses of Asymmetry
in Lateral Organs
Recently, several studies have investigated the genetic controls of symmetry in certain plant organs. The molecular studies have focused on the establishment of
transsectional symmetry in leaves, and radial symmetry in flowers. Also, studies on
the establishment of dorsiventrality in lateral organs and the consequences of this
on lateral organ development in insects and other animals have provided additional
insights into the mechanisms by which gene expression cascades can set up gradients and establish two opposing cell fates.
10.1 Studies in Leaves
Radially symmetric leaves in phan mutants are abaxialized while phab mutant leaves
are adaxialized (Waites et al. 1998; McConnell and Barton 1998). Based on the
phantastica, phabulosa, and lbll mutant phenotypes, we can hypothesize that both
adaxial and abaxial cell fates are necessary for proper margin and lamina development. These results also suggest that adaxial and abaxial ceIl fates are mutually
antagonistic.
PHANTASTICA mRNA accumulates in lateral organ primordia such as leaf
primordia and flower primordia (Fig. 4). Interestingly, this PHANTASTICA mRNA